1. Remote Radio Head technology, Centralized Base Transceiver Station (CBTS) and signal transmission
As illustrated in FIG. 1A, in mobile communication systems, a wireless access network is typically composed of Base Transceiver Stations (BTSs) and Base Station Controllers (BSCs) or Radio Network Controllers (RNCs) for controlling a plurality of BTSs. Wherein the BSC is mainly composed of a base band processing subsystem, a radio frequency (RF) subsystem, and antennas etc and is responsible for transmitting, receiving, and processing wireless signal, a BTS can cover various cells by means of a plurality of antennas, as illustrated in FIG. 1B.
In mobile communication systems, there are wireless network coverage problems that are more difficult to be solved with conventional BTS technologies, such as, indoor coverage of high-rise buildings, coverage hole, or the coverage of shadow zone. The RRH technology is a more efficient solution being proposed to solve the above problems. In the BTS system using RRH technology, the primary radio frequency units and antennas are installed in regions that are required to provide coverage, and are connected to other units in the BTS through wideband transmission lines.
This technology can be further developed to a CBTS technology that uses RRH technology. Compared with the conventional BTS, the CBTS using RRH technology has many advantages: the centralized structure allows to use several Micro-Cells to replace a Macro-Cell which is based on the conventional BTS, therefore it can be adapted to various wireless environment better, and enhance wireless performances such as system capacity and coverage etc; the centralized structure enables the replacement of soft handoff in the conventional BTS by softer handoff, therefore obtains additional processing gain; the centralized structure also makes expensive base band signal processing resources become a resource pool shared by several cells, therefore obtains the advantage of Statistic Multiplex, and also decreases system cost. The following patents disclose in detail some implementations about the CBTS using RRH technology, which are U.S. Pat. No. 5,657,374, filed on Mar. 23, 1995, entitled “Cellular system with centralized base stations and distributed antenna units”, and U.S. Pat. No. 6,324,391, filed on Jun. 28, 1999, entitled “Cellular communication with centralized control and signal processing”, which are hereby incorporated by reference.
As illustrated in FIG. 2, the CBTS system 200 using RRH technology is composed of a central channel processing subsystem 201 and a plurality of Remote Radio Units (RRUs) 2041, 2042, . . . , 204M, which are connected to each other through wideband transmission links or a network. The central channel processing subsystem 201 is mainly composed of a channel processing resource pool 202 and a signal route distribution unit 203, etc. The channel processing resource pool 202 is formed by stacking a plurality of channel processing units 2021, 2022, . . . , 202N together, and is used to perform base band signal processing, etc. The signal route distribution unit 203 dynamically distributes the channel processing resources in accordance with different cell traffics to achieve efficient share of a plurality of cells processing resources. The signal route distribution unit 203 can be constructed as separate equipment outside of the CBTS other than is located inside of the CBTS as illustrated in FIG. 2. The RRUs 2041, 2042, . . . , 204M are mainly composed of functional units such as radio frequency power amplifiers in transmit channel, low noise amplifiers in receive channel, and antennae, etc (not shown entirely). Typically, the links between the central channel processing subsystem 201 and the Remote Radio Units (RRUs) 2041, 2042, . . . , 204M can use transmission media such as optical fiber, copper cable, microwave, etc.
In the two BTS systems using RRH technology discussed above, the key problem to be solved is the wireless signal transmission between the RRUs and the main BTS. The main BTS in the two BTS systems using RRH technology discussed above refers to the generic term of the BTS units including base band processing units other than the radio frequency units. Typically, analog intermediate frequency or analog radio frequency signal transmission scheme is adopted, although it is easier to adopt analog signal transmission scheme, there will introduce disturbing components, for example noise, etc, in analog lines, and the signal modulation in the transmission will also introduce nonlinear distortion. In addition, the analog transmission may decrease the utilization of transmission line, and hamper the implementation of large capacity multiplex technology; therefore, it is difficult to adopt the analog transmission scheme in large scale networking.
To this end, the scheme of digital signal transmission is proposed in the following patent applications: Chinese patent application CN1464666, filed on Jun. 11, 2002, entitled “A soft BTS system based on remote fiber and its synchronization method”, and Chinese patent application CN1471331, filed on Jul. 2, 2003 (the priority date being Jul. 2, 2002), entitled “The BTS system in mobile communication”. Wherein the scheme of digital base band signal transmission is generally adopted in order to decrease the requirement for transmission band width as much as possible. However, patent application CN1464666 only disclosed the simple method of using the optical fiber to transmit digital I/Q (In-phase/Quadrature) base band signals between the RRU and the main BTS, that is, the digital I/Q base band signals are converted to serial data stream by means of parallel to serial conversion at the transmit end, and then transmitted to the receive end by an optical transmitter, while restored to the digital I/Q base band signals by means of serial to parallel conversion after received by the optical receiver at the receive end. Patent application CN1471331 proposed a transmission technology using Ethernet technology in physical layer, the technology uses continuous bit stream format specially defined instead of Ethernet MAC (Media Access Control) frame. At present, a corporation organization named CPRI (Common public Radio Interface) is also engaged in the standardization of the digital base band transmission between the RRU and the main BTS. This technology specification adopts a technology similar to that adopted in patent application CN1471331, that is, physical interface uses 1000MB or 10GB Ethernet standard, and upper layer uses a continuous bit stream format user-defined. But CPRI only supports star networking in the form of point to point, whereas CN1471331 can support the link converge based on hub.
On the other hand, SDH (Synchronous Digital Hierarchy) and OTN (Optical Transmission Network) based on such Wavelength Division Multiplex technology as DWDM (Dense Wavelength Division Multiplex)/CWDM (Coarse Wavelength Division Multiplexing) have been widely used in backbone network and wideband Metropolitan Area Network (MAN), but the existing technology of digital transmission between the RRU and the main BTS uses specific transport protocols and specification, and therefore, it is difficult to use the existing maturate wideband transmission resources in the existing telecommunication network, so the networking cost is increased. Moreover, there are problems, such as nonflexible networking and complicated maintenance and management, in the existing technology of digital transmission between the RRU and the main BTS.
2. Generic Framing Procedure (GFP)
Generic Framing Procedure (GFP) recommended jointly by ITU-T and ANSI is used to adapt the data stream in the form of block code or packet type to continuous byte synchronization transmission channel, typically for example the new technologies as SDH (Synchronous Digital Hierarchy) and OTN (Optical Transmission Network), the detailed technology specification thereof may refer to ITU-T G.7041 or ANSI T1X1.5/2000-024R3, which are hereby incorporated by reference. GFP can be classified into frame mapping GFP (GFP-F) that supports PDU (Protocol Data Unit) and transparent GFP (GFP-T) that supports block code. The GFP-F can be used in the adaptation of protocol packet as PPP (Point to Point Protocol), MPLS (Multi-Protocol Label Switching), and Ethernet MAC (Media Access Control), etc. And the GFP-T can be used to adapt block code character stream in 1000 MB Ethernet line, etc, directly, thus some application requirements for very little time delay can be satisfied, but the utilization of the GFP-T transmission bandwidth is lower than that of GFP-F transmission bandwidth.
In FIG. 3, a frame structure of GFP-T type is illustrated schematically. As shown in FIG. 3, the GFP-T frame is composed of a core header and a payload part, and the payload part includes a payload header, payload and a selectable payload FCS (Frame Check Sequence, shown by dashed line). The core header includes a PL1 field indicating the payload length and a core header error control field (cHEC); the cHEC is functioned as GFP frame delineation similar to ATM (Asynchronous Transfer Mode) Cell delineation, as well as provides error protection for the core header. The payload header indicates payload types and provides error protection by the cHEC, wherein Payload Type Identifier (PTI) indicates that the GFP-T frame carries user data when it is “000”, and indicates that the GFP-T frame carries client management information when it is “100”; payload FCS indicator (PFI) indicates whether there is the payload FCS; User Payload Identifier (UPI) and the PTI together indicate the types of user data or client management information in the payload. More particularly, now referring to FIGS. 4A, 4B, the values of User Payload Identifier (UPI) and the types of user data in GFP frame payload are shown therein, wherein the corresponding relations between the various user data in the GFP frame and the respective PTI when the GFP frame payload carries the user data are defined in FIG. 4A, for example, if PTI=000 and carrying the user data frame, UPI=“0000, 0001” indicates frame mapping Ethernet MAC; UPI=“0000,0010” indicates frame mapping PPP, etc. Similarly, the corresponding relations between various client management information in the GFP frame and the respective PTI when the GFP frame payload carries the client management information are defined in FIG. 4B, for example, if PTI=100E and carrying the client management frame, UPI=“0000, 0001” indicates client signal failure (loss of client signal); UPI=“0000, 0010” indicates client signal failure (loss of client character synchronization). Furthermore, in the GFP-T frame payload header, Extension Header Identifier (EXI) indicates the presence of a selectable extension header and its type, GFP Extension Header Identifier defined in the current standard is shown in FIG. 5, wherein EXI=“0000” indicates that there is no extension header, EXI=“0001” and EXI=“0010” are used in the applications of logic point to point (linear) and logic circle link; when EXI=“0001”, the definition of extension header defined by ITU-T is a little different from that defined by ANSI, wherein ITU-T has defined a channel identifier (CID) of one byte to support multiplexing a plurality of individual client signals (the maximum number being 256), whereas ANSI standard uses the high 4 bits of the byte to indicate destination port, the low 4 bits to indicate source port, the definition of ANSI is the same with that of ITU-T in function and in essence, although the two definitions are different literally. The payload in the GFP-T frame is super block with fixed length which is formed by 64B/65B code block according to certain sequence, as shown in FIG. 3, since the direct adaptation of the transparent GFP now uses block code character stream of a 8B/10B line code, 64B/65B code block includes user data character and control character, so a flag bit is used to indicate whether there is a control character in the 64B/65B code block, wherein the high 4 bits of the control character are used as the subsequent control character indication and the position indication of the control code in the original 8B/10B code stream, and the low 4 bits are used to transmit the control code itself.
3. Virtual Concatenation (VCAT) Technology
The STM-N/OTM-n standard transmission link of SDH/OTN is formed by multiplexing some typical Virtual Containers (VCs) with fixed rate according to certain multiplex rules. For example, the basic VCs of SDH includes VC-11, VC-12, VC-2, VC-3 and VC-4, and VC-4 can form four VCs with higher rate: VC-4-4c, VC-4-16c, VC-4-64c and VC-4-256c by means of sequential concatenation, as illustrated in table 1.
TABLE 1VC typeVC bandwidthVC payload bandwidthVC-111664Kbit/s1600Kbit/sVC-122240Kbit/s2176Kbit/sVC-26848Kbit/s6784Kbit/sVC-348.960Mbit/s48.384Mbit/sVC-4150.336Mbit/s149.760Mbit/sVC-4-4c601.344Mbit/s599.040Mbit/sVC-4-16c2405.376Mbit/s2396.160Mbit/sVC-4-64c9621.504Mbit/s9584.640Mbit/sVC-4-256c38486.016Mbit/s38338.560Mbit/s
The technology of using finite number of fixed rate VCs has simplified SDH multiplex design, and made it easier to realize Add/Drop, multiplex and digital cross connect, but since a plenty of padding are needed to adapt specific VC rate, the transmission efficiency is influenced. Whereas the Virtual Concatenation (VCAT) technology allows for providing more selections on transmission bandwidth by inversely multiplexing a plurality of VCs having the same rate, so the problems with transmission efficiency are solved, but since each VC arrives at the receive end through separate transmission paths, certain buffer is needed at the receive end to eliminate the difference due to transmission delay.
4. The Signal Transmission of Wireless BTS Based on RRH Technology
To solve the problems with wireless signal transmission between the RRU and the main BTS in the existing technology, the applicant of the present application also filed an invention patent application named “A method and system of signal transmission in Base Transceiver Station based on remote radio head”. In this application, the invention has proposed a technology of digital wireless signal transmission between the RRU and the main BTS. The proposed digital wireless signal transmission technology is compatible with the existing telecommunication transmission network technology, and can access to the existing SDH/OTN transmission network directly. Because the technology adopts the STM-N/OTM-n standard interface directly, the digital wireless signal transmission between the RRU and the main BTS can be realized without specific transmission network.
According to the patent application, since digital wireless signal data stream and in-band control signaling are mapped to the STM-N/OTM-n frame using the frame structure of transparent GFP (GFP-T), the SDH/OTN-based transmission is realized and the requirement for low transmission delay is satisfied. The in-band control signaling refers to the control, management, operation and maintenance data other than digital wireless signal data stream transmitted between the RRU and the main BTS. According to the patent application, multiple schemes of multiplexing the in-band control signaling and the digital wireless signal data stream are proposed, that is, the scheme of transmitting the in-band control signaling using CMF frame, the scheme of transmitting the in-band control signaling using control character, the multiplex scheme of mapping the in-band control signaling link layer packet to GFP-F frame, the GFP-T frame multiplex scheme of using the in-band control signaling as independent client signal, and the time division multiplex scheme of multiplexing the digital wireless signal data steam and the in-band control signaling, therefore the digital wireless signal data steam and the respective in-band control signaling can be transmitted simultaneously using the same transmit channel. Since the patent application is the extension of GFP-T protocol, most software and hardware designs of GFP-T can be used directly in the implementation, so that the implementation difficult is greatly reduced. This application as a whole is hereby incorporated by reference.
However, this application is mainly aimed at using single antenna, and in practical wireless BTS systems, more and more systems adopt multi-antenna technologies to obtain enhanced wireless performance, typically, such technologies as transmit diversity, receive diversity, Multiple-Input Multiple-Output (MIMO) and Smart Antenna or Antenna Array, etc. In the BTS system using remote radio head (RRH) technology and multi-antenna technology, to ensure strict time/phase relations between various antenna signals, the transmission delays to the CBTS for various signals are required to be the same.
Time delay compensation technology, as the method adopted in CPRI technology specification, requires measuring precisely the time delays of transmission links corresponding to various antennas, and it is only adapted to the networking with fixed time delay, such as point to point link, etc. . . . When intermediate transmission network node is present, the time delay will vary at random because of the vary of network traffic, so time delay measurement and time delay compensation should be performed on the transmission links corresponding to various antennas continuously, in doing so, the system is complicated, and strict time/phase relations between various antenna signals required by the multi-antenna technology are usually hard to be realized.